1. Field of the Invention
This invention generally relates to an image forming apparatus such as a copying machine, a printer or a facsimile apparatus for effecting image forming by an electrophotographic process or the like, and more particularly to an image forming apparatus in which a visible image (toner image) developed on an image bearing member by an electrophotographic process or the like is transferred to a transfer medium such as an intermediate transfer member.
2. Related Background Art
Heretofore, for example, in an image forming apparatus for forming an image on an image bearing member by an electrophotographic process, in order to eliminate the influence of an environmental change or a fluctuation with time to thereby keep the quality of image stable, process control such as exposure amount adjustment and potential control has been effected at electrostatic latent image forming and developing steps, etc.
Also, recently, from the advantage that the use of such an intermediate transfer member as shown in FIG. 1 of the accompanying drawings leads to the securability of transferability to various recording materials, instead of being directly transferred from an image bearing member to a recording material, there have been supplied in the market many image forming apparatuses using an intermediate transfer member. Such image forming apparatus using the intermediate transfer member have been proposed with the view of obtaining full-color image forming free of color misregister.
Briefly describing such an image forming apparatus, the image forming apparatus is provided with a photosensitive drum 1 as an image bearing member driven in the direction of arrow at a predetermined peripheral speed, and around the photosensitive drum 1, there are disposed a corona charger 2 as primary charging means, first developing means 7, i.e., a black developing device BK which is a fixed developing apparatus, second developing means 8 which is a rotary developing apparatus carrying a magenta developing device M, a yellow developing device Y and a cyan developing device C thereon, an intermediate transfer member 9 and cleaning means 11.
The photosensitive drum 1 is first uniformly charged by the corona charger 2, and a light image of a predetermined color is scanned thereon by image exposing means 5 such as a laser beam exposing apparatus, whereby electrostatic latent image forming is effected.
A latent image formed on the photosensitive drum 1 is visualized into a toner image by one of the developing devices of the first developing means 7 and the second developing means 8.
This visualized toner image on the photosensitive drum 1 is transferred to the intermediate transfer member 9. That is, in this example, the intermediate transfer member 9 which is an intermediate transfer belt movably passed over supporting rollers 9a–9d is brought into contact with the surface of the photosensitive drum 1 with a predetermined pressure force by a primary transfer roller 15 as primary transferring means at a nip section whereat it is moved substantially at the same speed and in the same direction as the photosensitive drum 1, and a voltage opposite in polarity to the charging polarity of the toner and primarily set in advance is applied to the primary transfer roller 15. Thereby, the toner image on the photosensitive drum 1 is transferred onto the intermediate transfer member 9.
The above-described process is repeated a plurality of times for each color, whereby a full-color image is formed on the intermediate transfer member 9. The full-color image formed on the intermediate transfer member 9 is collectively transferred to a recording material P by a secondary transfer roller 10 as secondary transferring means, whereby a full-color image is formed on the recording material P.
Heretofore, the intermediate transfer member 9 and the primary transfer roller 15, in order to suitably adjust the resistance values thereof, have been of a construction in which an electrically conductive substance such as carbon or a metal oxide is dispersed in an elastic material such as rubber.
Generally it is known in such material that the resistance value thereof fluctuates during manufacture and that the resistance value thereof greatly fluctuates with a change in the ambient environment.
When in such a situation, a primary transferring bias applied to the primary transfer roller 15 through the intermediate transfer member 9 is a constant voltage primarily set in advance as in the prior art, it is generally practiced to effect such control as makes a transfer voltage great and carry out the correction of the environment, for example, in a case where as under a low-temperature and low-humidity environment, the resistance of the intermediate transfer member 9 and the primary transfer roller 15 becomes high.
In order to achieve the optimization of such a transferring high voltage as described above, in the prior art, there has been proposed an adjusting method whereby a non-image portion potential area is formed on the photosensitive drum 1 in a pre-rotation section immediately before image forming so that a preset necessary current may be obtained for each environment, and at the timing whereat this non-image portion potential area has arrived at a position opposed to the primary transfer roller 15, a predetermined current is applied and a current-voltage characteristic is measured so that a necessary transferring current may be obtained, and on the basis of the measurement, a primary transferring high voltage during image forming is corrected. It is the substance described, for example, in Japanese Patent Application Laid-Open No. H8-194389.
However, in the process as described above wherein an image is formed on an image bearing member by a regular developing method by the use of a plurality of developing means, when dark portion potential (VD) and light portion potential (VL) on the image bearing member are controlled for each of the plurality of developing means so as to adjust developing contrast potential and non-image portion contrast potential, even if a voltage value is adjusted relative to the non-image portion potential, the dark portion potential (VD) at which the toner image is formed differs from one color to another and therefore, there has become necessary the correction control of the transferring voltage taking not only a constant voltage value in the non-image portion but also the set value of the dark portion potential (VD) into account, and this control has been complicated.
Also, the inventor has empirically confirmed the presence of the inconvenience that when a full-color image is to be formed on an image bearing member by the use of a plurality of developing means, if image forming is effected by the use of developing means of different developing types, for example, developing means of a magnetic non-contact developing type and developing means of a two-component developing type at a time, influences given to the image bearing member during developing (such as fog, adherence of a carrier and friction by contact given a peripheral speed ratio) differ and therefore, even if the same bias value is applied from a primary transferring high voltage source is applied to regions having received the influences of the respective developing means, there occurs the phenomenon that flowing voltage-current characteristics differ from one another.
Also, the inventor has confirmed that the above-described influences of the difference between the developing means upon the image bearing member are also affected by changes in an electrical characteristic by the environmental fluctuations of developers, the intermediate transfer member and the transfer roller.